PROJECT SUMMARY Anxiety disorders are among the most common psychiatric conditions in children and adolescents with a prevalence of more than 10%. Left untreated, pediatric patients with anxiety disorders suffer physical, emotional, academic, and social impairment, and these disorders increase the likelihood of secondary anxiety disorders, major depressive disorder and other psychiatric conditions in adulthood. Selective-serotonin reuptake inhibitors (SSRIs) are the first-line psychopharmacologic treatment for these conditions, but may take up to 8 weeks to produce responses?which only occur in 50-60% of youth. Further, SSRI-related adverse effects emerge early in the course of treatment, decrease the likelihood of success and increase the likelihood of medication discontinuation. These significant side effects include gastrointestinal symptoms and activation?a hyperarousal event characterized by specific symptoms including an increase in activity, impulsivity, disinhibition, restlessness, and insomnia. Today, there is no way to predict, based on clinical or biological characteristics which children and adolescents will respond to an SSRI or who will develop treatment-limiting adverse effects?a significant concern of patients and their families. The goal of this proposal is to advance the treatment of pediatric anxiety disorders by examining predictive markers of SSRI treatment response and SSRI-related gastrointestinal symptoms and activation. This proposal builds upon our significant finding that 5 lipid classes derived from plasma extracellular vesicles (EVs) predict SSRI response in anxious youth. This application proposes to replicate our preliminary study in a large, multi- site, double-blind placebo-controlled trial and utilizes synchronized methods for biospecimen collection and outcome measurement. This proposal will also evaluate a specific subpopulation of EVs?L1CAM(+) EVs? which are enriched in cargoes of neuronal origin, providing a targeted, non-invasive assessment of the molecular milieu of the CNS. The line of inquiry is novel to precision medicine/child and adolescent mental health, and has the potential to propel the treatment of anxiety disorders in youth while increasing the safety of SSRI treatment, by predicting SSRI-related side effects.